I am not disagreeing in principle that adding HDMI could be a benefit. However, it does add to the complexity that many people would not want. As it stands now, the TV acts as the hub with all sources going to the TV, and the Playbar is just the audio output from the TV. Instead, using HDMI:
1) All of the sources would have to run to the Playbar or some sort of splitter/hub 1st, then to the TV. In other words, it would be more "AVRish".
--OR--
2) As you indicate, use HDMI ARC and just have the Playbar use 1 of the TV's HDMI ports. Sounds great, but in reality the vast majority of TVs are already short on HDMI ports! So many people would have to add a splitter/hub.
Anyhow, I am OK with my setup as my TV does pass Dolby 5.1 to the optical output. And, everything I have is Dolby 5.1 as opposed to DTS or other "more advanced" signals. So the current capabilities of Sonos suit me well. I actually only use a single box for 100% of my viewing, the AppleTV 4, and I love my simple setup in my family room. (I have a traditional AVR 5.1 setup in my basement theater.)
I'm with you there, in terms of wanting simplicity. Out of curiosity, what TV do you have that passes Dolby 5.1 to the optical out? Do all models of this brand do it?
I don't the HDMI/HDMI ARC system is so complex. You're right that it's not perfect in terms in input management, but HDMI-CEC does help there a lot.
For my it's more about price. At $700, the Sonos Playbar is on the expensive-side of sound bars. For that much money, I expect more features and I want it to support higher-quality connectivity standards. I expect better from a company like Sonos. They know their stuff, they make a good product, and they know who they are competing with.
For the same exact price, the Bose SoundTouch 300 offers all the same features of the Sonos Playbar but with support for HDMI or optical. It also does Wifi streaming, controlled by an app, connectors to Spotify, Pandora, SeruisXM, Amazon, Deezer, etc. (Apple Music missing for now). It also does bluetooth though. It also has an optional wireless sub and wireless satellites you can add for added benefit.
Also, using HDMI ARC does not change the number of ports used. Look, assuming a TV has 2 HDMI ports, below shows that two HDMI ports on the TV are used either way. Also, the input controls are still done on the TV.
With optical -
TV HDMI1: Set-top box
TV HDMI2: Game console
TV Optical: Audio of whatever input is selected to sound bar
SB Optical: Audio of whatever input is selected from tv
With HDMI ARC -
SB HDMI in: Set-top box
SB HDMI out: using ARC: video-out of set-top box to TV or audio-in of game console from TV, based on input on TV
TV HDMI1: using ARC: video-in of set-top box, or audio-out of game console, based on input on TV
TV HDMI2: game console
TV Optical: (nothing)
It seems messier, but there is actually no difference in the number of wires, and the input controls are still handled by the TV.